Cite this: DOI: 10.1039/c3ce27082c

Syntheses, structures, and luminescence of fourlanthanide metal–organic frameworks based onlanthanide-oxide chains with C2- or C3-symmetrictrigonal-planar polycarboxylate ligands3

Received 23rd December 2012,Accepted 14th April 2013

DOI: 10.1039/c3ce27082c

www.rsc.org/crystengcomm

Xiutang Zhang,ab Liming Fan,ab Zhong Sun,b Wei Zhang,b Weiliu Fan,a Liming Suna

and Xian Zhao*a

Solvothermal reactions of two trigonal-planar ligands and lanthanide metal cations of LnIII afford four new

coordination polymers (CPs), {[LnNa0.33H0.67(PBPP)2]?2H2O}n (Ln = Pr for 1, Gd for 2, Ce for 3) and

{[Sm(TATB)(H2O)]}n (4) (H2PBPP = 4-phenyl-2,6-bis(49-carboxyphenyl)pyridine, H3TATB = 4,49,499-s-triazine-

2,4,6-tribenzoic acid). Their structures have been determined by single-crystal X-ray diffraction analyses

and further characterized by elemental analyses, IR spectra, powder X-ray diffraction (PXRD) and

thermogravimetric (TG) analyses. Complexes 1–3 are isomorphous and exhibit an unprecedented (4,6,6)-

connected 3D architecture, which is built up from lanthanide-oxide chains. Complex 4 shows a 3D (6,6)-

connected net with (44.67.84)6(48.67) topology, in which TATB32 is extremely unsymmetrical due to three

metal-oxide chains in the directions of [101], [101] and [110]. Moreover, their luminescent properties have

been investigated.

Introduction

The design and synthesis of lanthanide metal–organic frame-works (MOFs) has attracted upsurging research interest notonly because of their appealing structural and topologicalnovelty but also owing to their exceptional optical andluminescence properties arising from f–f transitions of thelanthanide ions with a narrow bandwidth.1–4 So far, a largenumber of lanthanide MOFs with various structures andinteresting properties have been obtained but the design andcontrol over lanthanide-based frameworks appears to be aformidable task primarily due to the high coordinationnumber and diverse coordination geometry of lanthanideions.5–7

One outstanding aspect of the lanthanides is that they haveunique luminescence features, such as high luminescencequantum yields, narrow bandwidths, long-lived emissions and

large Stokes shifts, due to the Laporte-forbidden f–f transi-tions.8,9 The 4f orbitals of LnIII are relatively insensitive to theligand field and these optical bands are line-like andcharacteristic of each metal. As a consequence of the parityrule (and sometimes due to the change in spin multiplicity),these absorptions have very low molar absorptivities, makingthe direct excitation of the metals very inefficient unless high-power laser excitation is utilized. With organic and inorganiccomponents involved, coordination networks are certainly verypromising as a multifunctional luminescent material becauseboth the inorganic units and the organic moieties can provideplatforms to generate luminescence, while metal–ligandcharge transfer or ligand–metal charge transfer relatedluminescence within the MOFs can add other dimensionalluminescent functionalities.10,11

Thus, these considerations inspired us to explore newlanthanide coordination frameworks with C2- or C3-symmetrictrigonal-planar polycarboxylate ligands of 4-phenyl-2,6-bis(49--carboxyphenyl)pyridine (H2PBPP) and 4,49,499-s-triazine-2,4,6-tribenzoic acid (H3TATB). Compared to rigid 1,3-benzenedi-carboxylic acid and 1,3,5-benzenetricarboxylic acid, H2PBPPand H3TATB are longer and more flexible ligands, whichpossess two interesting characteristics: (i) they have two orthree carboxyl groups that may be completely or partiallydeprotonated, inducing rich coordination modes and allowinginteresting structures with higher dimensionalities, (ii) twosets of separated carboxyl groups can form different dihedral

aState Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,

China. E-mail: zhaoxian@icm.sdu.edu.cnbAdvanced Material Institute of Research, College of Chemistry and Chemical

Engineering, Qilu Normal University, Jinan, 250013, China.

E-mail: xiutangzhang@yahoo.com.cn

3 Electronic supplementary information (ESI) available: Crystallographic data inCIF format, related structure of compound 1–4, powder X-ray diffraction (PXRD)patterns, thermogravimetric analysis (TGA) and IR spectra for 1–4 are availablefree of charge via the internet. CCDC 915817–915821 (complexes 1–5. For ESIand crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce27082c

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angles through the rotation of C–C single bonds and thus theymay ligate metal centers in different orientations. Thesecharacteristics may lead to cavities, interpenetration, helicalstructures and other novel motifs with unique topologies.12–14

Taking account of these, recently we began to assembleH2PBPP, H3TATB and different lanthanide ions into polymericcomplexes under solvothermal conditions and anticipated thatthe rich structural features stored in H2PBPP and H3TATB willinduce novel polymeric structures. In this paper, we report thesyntheses and characterizations of four novel Ln-MOFs builton lanthanide-oxide chains, {[LnNa0.33H0.67(PBPP)2]?2H2O}n

(Ln = Pr for 1, Gd for 2, Ce for 3), {[Sm(TATB)(H2O)]}n (4).Interestingly, the single-crystal X-ray diffraction analyses revealthat three-dimensional (3D) architectures of 1–4 are built upfrom lanthanide-oxide chains. Moreover, the luminescentproperties of compounds 1–4 have been studied.

Experimental section

Materials and physical measurements

All the chemicals were purchased from Jinan Henghua Sci. &Tec. Co. Ltd. without further purification. IR spectra weremeasured on a Nicolet 740 FTIR Spectrometer in the range of400–4000 cm21. Elemental analyses were carried out on a CEinstruments EA 1110 elemental analyzer. X-ray powderdiffractions were measured on a Panalytical X-Pert prodiffractometer with Cu–Ka radiation. Thermogravimetricanalyses (TGA) were performed under air conditions fromroom temperature to 1200 uC with a heating rate of 10 uCmin21 on a Perkin-Elmer TGA-7 thermogravimetric analyzer.Fluorescent data were collected on an Edinburgh FLS920TCSPC system.

Synthesis of {[PrNa0.33H0.67(PBPP)2]?2H2O}n (1). A mixture ofH2PBPP (0.20 mmol, 0.050 g), praseodymium(III) nitratehexahydrate (0.20 mmol, 0.087 g), oxalic acid (0.30 mmol,0.027 g), NaOH (0.40 mmol, 0.016 g), 8 mL of H2O and 2 mL ofDMF was placed in a Teflon-lined stainless steel vessel andheated to 170 uC for 3 days, followed by slow cooling (a descentrate of 10 uC h21) to room temperature. Green block crystals of1 were obtained. Yield of 73% (based on Pr). Anal. (%) calcd.for C50H34.67N2Na0.33O10Pr (972.03): C, 61.78; H, 3.59; N, 2.88.Found: C, 60.91; H, 3.62; N, 2.65. IR (KBr pellet, cm21): 3407(m), 3067 (m), 2793 (w), 1653 (m), 1589 (s), 1547 (s), 1513 (s),1387 (s), 1369 (s), 1298 (m), 1091 (w), 1039 (w), 836 (m), 812(m), 711 (w).

Synthesis of {[GdNa0.33H0.67(PBPP)2]?2H2O}n (2). The samesynthetic procedure as for 1 was used except thatpraseodymium(III) nitrate hexahydrate was replaced bygadolinium(III) nitrate hexahydrate (0.20 mmol, 0.090 g),giving colorless block crystals. Yield of 67% (based on Gd).Anal. (%) calcd. for C50H34.67GdN2Na0.33O10 (988.38): C, 60.76;H, 3.54; N, 2.83. Found: C, 59.62; H, 3.43; N, 2.77. IR (KBrpellet, cm21): 3476 (m), 3063 (m), 2826 (w), 1712 (w), 1583 (s),1546 (s), 1517 (s), 1391 (s), 1365 (s), 1298 (m), 1137 (w), 978 (w),837 (w), 723 (m).

Synthesis of {[CeNa0.33H0.67(PBPP)2]?2H2O}n (3). The samesynthetic procedure as for 1 was used except thatpraseodymium(III) nitrate hexahydrate was replaced bycerium(III) nitrate hexahydrate (0.20 mmol, 0.087 g), givingcolorless block crystals. Yield of 54% (based on Ce). Anal. (%)calcd. for C50H34.67CeN2Na0.33O10 (971.24): C, 61.84; H, 3.60; N,2.88. Found: C, 60.89; H, 3.71; N, 2.73. IR (KBr pellet, cm21):3407 (s), 3063 (m), 1612 (s), 1597 (s), 1547 (s), 1507 (s), 1393 (s),1371 (vs), 1311 (m), 1143 (w), 1037 (w), 846 (m), 767 (m), 719(w).

Synthesis of {[Sm(TATB)(H2O)]}n (4). A mixture of H3TBTA(0.10 mmol, 0.046 g), samarium(III) nitrate hexahydrate (0.20mmol, 0.089 g), oxalic acid (0.30 mmol, 0.027 g), NaOH (0.50mmol, 0.020 g), 13 mL of H2O, and 2 mL of DMF was placed ina Teflon-lined stainless steel vessel and heated to 170 uC for 3days, followed by slow cooling (a descent rate of 10 uC h21) toroom temperature. Colorless block crystals of 4 were obtained.Yield of 48% (based on Sm). Anal. (%) calcd. forC24H14N3O7Sm (606.73): C, 47.51; H, 2.33; N, 6.93. Found: C,47.21; H, 2.17; N, 6.69. IR (KBr pellet, cm21): 3438 (m), 3126(m), 2813 (w), 1631 (vs), 1550 (s), 1446 (m), 1389 (m), 1332 (m),1309 (s), 1013 (w), 883 (w), 797 (s), 693 (w).

X-ray crystallography

Intensity data collection was carried out on a Siemens SMARTdiffractometer equipped with a CCD detector using Mo–Kamonochromatized radiation (l = 0.71073Å) at 293(2) K. Theabsorption correction was based on multiple and symmetry-equivalent reflections in the data set using the SADABSprogram based on the Blessing method. The structures weresolved by direct methods and refined by full-matrix least-squares using the SHELXTL package.15 All non-hydrogenatoms were refined anisotropically. The hydrogen atoms,except those for the water molecules, were generated geome-trically with fixed isotropic thermal parameters and includedin the structure factor calculations. The hydrogen atoms forthe water molecules were not included in the refinement. Fivecarbon atoms in one phenyl ring of PBPP22 in compounds 1–3are disordered and were refined with split positions and anoccupancy ratio of 1 : 1. The crystallographic data forcompounds 1–4 are given in Table 1. Selected bond lengthsand angles are listed in Table 2. Complexes 1–4 have thefollowing depository number: CCDC 915820 for 1, 915819 for2, 915817 for 3, 915821 for 4.

Results and discussion

Synthesis and characterization

Compounds 1–4 were obtained by solvothermal reactions ofH2PBPP or H3TATB and the related lanthanide salts in mixedsolvents of DMF and H2O at 170 uC. All the compounds areinsoluble in water and common organic solvents includingchloroform, toluene, acetonitrile, methanol and ethanol.

The IR spectra of 1–3 are similar. The absorption bands inthe range of 3400–3500 cm21 for 1–3 can be attributed to thecharacteristic peaks of water O–H vibrations. The vibrations atca. 1530 and 1565 cm21 correspond to the asymmetric and

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symmetric stretching vibrations of the carboxylate groups,respectively. The absence of strong absorption bands from1300 to 1600 cm21 proves that the H2PBPP ligand iscompletely deprotonated. For 4, the absorption bands ataround 3450 cm21 can be attributed to the characteristic peaksof water O–H vibrations. The vibrations in the range of 1250–1650 cm21 correspond to the asymmetric and symmetricstretching vibrations of the carboxylate groups of TATB32,respectively.

Structure descriptions of {[LnNa0.33H0.67(PBPP)2]?2H2O}n (Ln =Pr for 1, Gd for 2, Ce for 3)

The single-crystal X-ray diffraction analyses reveal that com-plexes 1–3 are isomorphous and crystallize in the rhombohe-dral system, R3 space group, therefore herein, only thestructure of 1 will be discussed as a representation. As canbe seen from Fig. 1, there is one PrIII ion (the occupation ratiosfor Pr(1) and Pr(2) are 1 : 3, 2 : 3, respectively), one third of aNaI ion, two partly deprotonated PBPP22 ligands and twolattice water molecules. Two different kinds of PrIII ions atspecial positions exhibit completely different coordination

Table 1 Crystal data for 1–4

Compound 1 2 3 4

Empirical formula C50H34.67N2Na0.33O10Pr C50H34.67GdN2Na0.33O10 C50H34.67CeN2Na0.33O10 C24H14N3O7SmFormula weight 972.03 988.38 971.24 606.73Crystal system Rhombohedral Rhombohedral Rhombohedral OrthorhombicSpace group R3 R3 R3 Fddda (Å) 29.5657(12) 29.6966(13) 29.5404(12) 22.304(3)b (Å) 29.5657(12) 29.6966(13) 29.5404(12) 28.721(4)c (Å) 14.8222(11) 14.9618(12) 14.9218(12) 29.158(4)a (u) 90 90 90 90b (u) 90 90 90 90c (u) 120 120 120 90V (Å3) 11 220(7) 11 426(9) 11 276(8) 18 678(4)Z 9 9 9 32Dcalcd (Mg m23) 1.295 1.293 1.287 1.726m (mm21) 1.035 1.363 0.966 2.565T (K) 293(2) 293(2) 293(2) 296(2)Rint 0.0343 0.0772 0.0230 0.0550Final R indices[I . 2s(I)] R1 = 0.0360 R1 = 0.0732 R1 = 0.0332 R1 = 0.0403

wR2 = 0.1006 wR2 = 0.2235 wR2 = 0.0984 wR2 = 0.1308R indices (all data) R1 = 0.0422 R1 = 0.1196 R1 = 0.0412 R1 = 0.0512

wR2 = 0.1058 wR2 = 0.2671 wR2 = 0.1073 wR2 = 0.1373Gof 1.000 1.001 1.001 0.997

R1 = S||Fo| 2 |Fc||/S|Fo|, wR2 = [Sw(Fo2 2 Fc

2)2]/Sw(Fo2)2]1/2.

Table 2 Selected bond lengths (Å) and angles (u) for 1–4

Complex 1Na(1)–O(4) 2.401(2) Pr(1)–O(1) 2.619(2) Pr(1)–O(2) 2.658(3) Pr(2)–O(3) 2.571(2)Pr(2)–O(4) 2.518(2) O(1)–Pr(1)–O(2) 48.923(8) O(4)–Pr(2)–O(3) 51.01(7) O(4)–Na(1)–O(4)#1 75.39(8)O(3)–Pr(2)–O(3)#1 117.28(3) O(4)–Pr(2)–O(4)#1 71.32(8) O(3)–Pr(2)–O(4)#1 120.12(8)Symmetry codes: #1 2y, x 2 y, z.

Complex 2Gd(1)–O(1) 2.648(8) Gd(1)–O(2) 2.696(8) Gd(2)–O(4) 2.563(8) Gd(2)–O(3) 2.615(8)Na(1)–O(4) 2.427(8) O(1)–Gd(1)–O(2) 48.3(3) O(4)–Gd(2)–O(3) 49.8(3) O(2)–Gd(1)–O(2)#1 60.25(4)O(2)–Gd(1)–O(2)#3 119.75(3) O(1)–Gd(1)–O(2)#4 71.1(3) O(2)–Gd(1)–O(2)#4 60.24(3)Symmetry codes: #1 y + 1/3, 2x + y + 2/3, 2z 2 1/3. #2 2y + 1, x 2 y, z. #3 2x + y + 1, 2x + 1, z. #4 x 2 y + 1/3, x 2 1/3, 2z 2 1/3.

Complex 3Ce(1)–O(4) 2.645(2) Ce(1)–O(3) 2.662(3) Ce(2)–O(4) 2.459(2) Na(1)–O(2) 2.394(2)O(4)–Ce(1)–O(3) 48.58(8) O(4)–Ce(1)–O(3)#1 71.45(9) O(3)–Ce(1)–O(3)#1 60.280(13) O(3)–Ce(1)–O(4)#2 77.38(11)O(3)–Ce(1)–O(3)#3 119.720(13) O(4)–Ce(1)–O(3)#4 77.36(11) O(4)–Ce(2)–O(4)#3 71.03(8)Symmetry codes: #1 y + 1, 2x + y + 1, 2z + 2. #2 2x + 2, 2y, 2z + 2. #3 2y + 1, x 2 y 2 1, z. #4 x 2 y, x 2 1, 2z + 2.

Complex 4Sm(1)–O(1) 2.324(9) Sm(1)–O(1W) 2.468(1) Sm(1)–O(3)#3 2.353(9) Sm(1)–O(4)#2 2.344(9)Sm(1)–O(5)#4 2.380(8) Sm(1)–O(6)#5 2.411(8) O(1W)–Sm(1)–O(5)#4 144.5(5) O(2)–Sm(1)–O(1W) 74.9(4)O(1)–Sm(1)–O(2)#1 83.0(3) O(2)–Sm(1)–O(4)#2 75.5(3) O(2)–Sm(1)–O(3)#3 149.0(4) O(2)–Sm(1)–O(5)#4 129.5(3)O(2)–Sm(1)–O(6)#5 87.8(3) O(2)–Sm(1)–O(5)#5 127.7(3) O(1W)–Sm(1)–O(1)#1 78.9(5)Symmetry codes: #1 2x + 9/4, y, 2z + 9/4. #2 2x + 7/4, 2y + 3/4, z. #3 x + 3/4, y 2 1/4, 2z +2. #4 2x + 2, y + 1/4, z + 1/4. #5 x + 1/2, 2y + 1/4,2z + 7/4

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environments: Pr(1), dodeca-coordinated; Pr(2), nona-coordi-nated. The bond lengths of Pr–O are in the range of 2.443(2)–2.658(3) Å and are normal compared to the reported ones.16

The neighbouring metal ions along the c axis are staggeredinto 1D chains consisting of {Pr3Na} subunits with a –Pr(2)–O–Pr(1)–O–Pr(2)–O–Na(1)–O–Pr(2)– arrangement (Fig. 2 and S1,ESI3). The Pr(1)…Pr(2), Pr(2)…Na(1), and Pr(1)…Na(1) distancesare 3.849, 3.562, and 7.412 Å, respectively. These 1D chains arefurther linked via two deprotonated carboxyl groups of PBPP22

to form a petal-like 3D structure consisting of [Pr3Na(PBPP)]4

cages based on {Pr3Na} subunits (Fig. 3). It is noteworthy thatthe non-coordinating phenyl groups of the PBPP22 ligands,distributed in turn at both sides of the [Pr3Na(PBPP)]4 SUBs,may be one important factor to control the whole framework.If the guest water molecules are omitted, the void volume of 1is 21.9% of the crystal volume (2461.3 out of the 11 220.7 Å3

unit cell volume), calculated by PLATON.17

To better understand the structure of 1, the topologicalanalysis approach is employed. From the topological point ofview,18 the PBPP22 ligands act as 4-connected nodes and thePrIII and NaI ions act as 6-connected nodes, giving rise to atrinodal (4,6,6)-connected net with a point symbol of(42.63.8)3(46.66.83)2, shown in Fig. 4.

Structure descriptions of {[Sm(TATB)(H2O)]}n (4)

The single-crystal X-ray diffraction analysis reveals thatcomplex 4 crystallizes in the orthorhombic system, Fddd spacegroup, an isostructure of the EuIII complex reported by Zhanget al.19 The asymmetric unit of 4 contains one crystal-lographically independent SmIII ion, one TATB32 ligand andone associated water molecule, shown in Fig. 5. In 4, the threecarboxyl groups of TATB32 are completely deprotonated andshow the coordination mode: syn–anti m2-g1:g1, m2-g2:g1. EachSmIII ion is octa-coordinated by seven carboxylate oxygenatoms from six TATB32 ligands and one coordinated watermolecule. The Sm–O bond distances range from 2.324(9) to2.802(9) Å. Each two symmetrical neighboring SmIII ions arebridged by two m2-g1:g1 and two m2-g2:g1 carboxylate groupsfrom four TATB32 ligands to generate dinuclear Sm2 SBUswith a Sm…Sm distance of 4.054 Å (Fig. S3, ESI3), which arefurther extended into three 1D metal carboxyl chains along thethree [101], [101], and [110] directions via two m2-carboxylategroups (Fig. 6), which result in the large distortion of TATB32,different from the reported ones. The dihedral angles betweenthe central triazine and the three benzene rings are 2.45, 4.29and 19.17u (Fig. S4, ESI3) and the dihedral angles among thethree benzene rings are 6.74, 19.28 and 19.92u. This dihedraldata indicates that the TATB32 was extremely unsymmetricaland not as flat as the reported ones.

Fig. 1 Perspective view of the coordination environment of PrIII in 1 (purplepolyhedron: PrO12; turquoise polyhedron: PrO9; green polyhedron: NaO6).Hydrogen atoms are omitted for clarity. Symmetry codes: A: x 2 1/3, y + 1/3, z +1/3; B: 2y + 1, x 2 y 2 1, z; C: y + 1, 2x + y + 1, 2z + 2; D: 2x + y + 2, 2x + 1, z; E:2x + 2, 2y, 2z + 2; F: x 2 y, x 2 1, 2z + 2; G: 2y + 4/3, x 2 y 2 1/3, z 2 1/3; H:2x + y + 4/3, 2x + 2/3, z 2 1/3; I: x + 1/3, y 2 1/3, z 2 1/3; J: 2x + 4/3, 2y + 2/3, 2z + 5/3; K: 2x + y + 1, 2x + 1, z; L: x 2 y + 1/3, x 2 1/3, 2z + 5/3; M: 2y + 1,x 2 y, z; N: y + 1/3, 2x + y + 2/3, 2z + 5/3; O: 2x + 5/3, 2y + 1/3, 2z + 4/3.

Fig. 2 The view of the 1D infinite [Pr3Na(COO)12] chain along the bc plane (Pr1:purple polyhedron, Pr2: turquoise polyhedron, Na1: green polyhedron).

Fig. 3 View of the 3D frameworks and the contained octahedral cage structure.

Fig. 4 Views of the (4,6,6)-connected net with unprecedented(42.63.8)3(46.66.83)2 topology.

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These 1D [Sm2(COO)6]n chains crossed layer by layer,forming an unprecedented nanoscale 3D net consisting offour TATB32 ligands (Fig. 7). If the guest water molecules areomitted, PLATON calculations show that the void volume of 4is 11.1% of the crystal volume (2058.8 out of the 18 678.4 Å3

unit cell volumes). The low porosity can be attributed to thehigher coordination number of LnIII ions. From the viewpointof the structural topology, both the TATB32 ligand and theSmIII cation act as 6-connected nodes and the whole 3Dstructure exhibits binodal (6,6)-connected nets with a pointsymbol of (44.67.84)6(48.67) (Fig. 8).

X-ray powder diffraction analyses and thermal analyses

In order to check the phase purity of these complexes, thePXRD patterns of the title complexes were checked at roomtemperature. As shown in Fig. S9, ESI,3 the peak positions ofthe simulated and experimental PXRD patterns are inagreement with each other, demonstrating the good phasepurity of the complexes. For complexes 1–3, the differences inthe intensity in the PXRD patterns may be due to the preferredorientation of the crystalline powder sample.20

Thermogravimetric analysis (TGA) was performed on samplesof 1–4 under a N2 atmosphere with a heating rate of 10 uCmin21, shown in Fig. S10, ESI.3 For complexes 1–3, the firstweight loss of 3.97% (calc. 3.70%) for 1, 3.86% (calc. 3.64%) for

2 and 3.62% (calc. 3.71%) for 3 occurred below 225 uC. Thesecond weight loss observed around 400 uC is attributed to therelease of the PBPP22 ligands. The TGA curve of 4 shows thatthe first weight loss is 3.23% (calc. 2.97%) and the hostframework is stable up to ca. 512 uC.

Photoluminescent investigation

The fluorescence spectra of 1–4 were examined in the solidstate at room temperature and shown in Fig. 9. Compound 1exhibits characteristic PrIII emissions (excited at 431 nm). Theemission bands at 613, 502, 482, 460 nm arise from the 1D2 A1G4, 1D2 A 3F4, 1D2 A 2F4 and 1D2 A 1F4 transitions,respectively. For compounds 2 and 3, the emission bands at382 nm and 403 nm are attributed to the p*–n transitions ofPBPP32. Compound 4 displays narrow and characteristicluminescence due to 4G5/2 A 6F11/2, 4G5/2 A 6F9/2, 4G5/2 A6F7/2, 4G5/2 A 6F5/2, 4G5/2 A 6F3/2, and 4G5/2 A 6F1/2 transitionsresulting from the SmIII ion, with emission bands at 680, 641,612, 592, 493, 452 nm.21

Conclusions

In summary, four new Ln-MOFs based on lanthanide-oxidechains have been hydrothermally synthesized in the presenceof C2- or C3-symmetric polycarboxylate ligands of H2PBPP andH3TATB. Complexes 1–3, built up from lanthanide-oxidechains, exhibit an unprecedented (4,6,6)-connected 3D archi-

Fig. 6 The three parallel [Sm2(COO)6] chains linked by one TATB32 ligand (left)and the 3D network constructed from the cross stacking of 1D chains (right)(turquoise polyhedron: PrO8).

Fig. 7 The 3D high-connected frameworks possessing the nanoscale tetrahedralcage.

Fig. 8 Views of the (6,6)-connected net with unprecedented (44.67.84)(48.67)topology.

Fig. 5 Coordination environment of the SmIII in 4 (turquoise polyhedron: PrO8).Hydrogen atoms are omitted for clarity. Symmetry codes: A: 21/2 + x, 1/4 2 y,7/4 2 z; B: 9/4 2 x, y, 9/4 2 z; C: 23/4 + x, 1/4 + y, 2 2 z; D: 7/4 2 x, 3/4 2 y, z;E: 2 2 x, 21/4 + y, 21/4 + z; F: 3/4 + x, y 2 1/4, 2 2 z; G: 2 2 x, 1/4 + y, 1/4 + z;H: 1/2 + x, 1/4 2 y, 7/4 2 z; I: 5/2 2 x, 1/2 2 y, 2 2 z.

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tecture with a Point Schlafli symbol of (42.63.8)3(46.66.83)2.Complex 4 shows a 3D (6,6)-connected net with (44.67.84)6

(48.67) topology. These four inorganic clusters, quite rareamong known MOFs, significantly expand the pool ofinorganic building blocks and are clearly useful for theconstruction of porous MOFs with a high gas uptake capacity.

Acknowledgements

The work was supported by financial support from the NaturalScience Foundation of China (Grant No. 91022034, 51172127,21101097), the Natural Science Foundation of ShandongProvince (JQ201015, ZR2010BQ023) and the Qilu NormalUniversity is acknowledged.

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Fig. 9 Excitation (left) and emission (right) spectra of complexes 1–4 in the solidstate at room temperature.

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